Oxide glasses doped with rare earth ions have been found to have many valuable applications. For example, they are useful in lasers, optical switches, optical amplifiers and have anti-glare applications. Vitreous SiO2 glass is a good host for rare earth ions because of its high transparency, its low coefficient of thermal expansion, and its excellent chemical durability.
SiO2 glass doped with rare earth ions has a light absorption property in the visible light range. Because of this absorption property, SiO2 glass doped with rare earth ions can be used as a color filtering glass or a selective light filtering glass or, when the rare earth is Nd, a contrast enhancement glass for displays. SiO2 glasses doped with Nd3+ ions and SiO2 glass doped with Er3+ ions have found use as anti-glare glasses.
Existing rare earth doped glasses have encountered some problems due to the low solubility of rare earth ions in SiO2 causing concentration quenching which lowers the optical efficiency of the glass. At higher percentages of rare earth ions, problems with phase separation are encountered due to the low solubility of rare earth ions in SiO2. Phase separation lowers the transmittance of the glass because of the scattering of visible light. Concentration quenching is another problem associated with rare earth doped SiO2 glasses. Concentration quenching of SiO2 glass doped with rare earth ions has been reported to be due to the clustering of rare earth elements. In order to increase the solubility of rare earth ions in SiO2 glass, Al3+ ions are often co-doped. However, these glasses still suffer concentration quenching when the concentration of rare earth ions reaches useful levels. For example, a problem with the prior art SiO2 glass doped with rare earth elements is that concentration quenching generally becomes significant when the amount of rare earth oxide exceeds 1 mol %.
Neodymium containing glasses are known to be useful in preventing glare and intense light. These glasses have found use in protecting against the temporary decline in eye sight caused by intense light, for example, from a car's headlights or sunlight coming through a windshield. This effect is due to neodymium's absorption of visible light which human eyes are sensitive to at around 580 nm.
The problems associated with rare earth glasses such as concentration quenching and low solubility of the rare earths leading to inhomogeneous distributions have limited the usefulness of these glasses.